1. Field of the Invention
The invention relates to circuit boards (CBs), e.g., printed circuit boards, imprinted boards, and any other support substrate. More particularly, to an apparatus and method for improving coupling between signals routed across plane discontinuities on CBs.
2. Description of the Related Art
CBs are typically manufactured with a plurality of layers, each layer being permanently affixed to an adjoining layer through a structural, non-conductive material. FIG. 1 is an example of a four layer CB 100. Referring to FIG. 1, the first and fourth layers 112 and 124, respectively, are signal trace layers on which signal lines are routed. The second and third layers 116 and 120, respectively, are voltage reference layers, e.g., the second layer 116 is a power layer and the third layer 120 is a ground layer. The power layer 116 and ground layer 120 are affixed to a core 118 comprised of a fiberglass mesh material such as FR4. The first signal layer 112 and the power layer 116 and the second signal layer 124 and the ground layer 120 each sandwich a pre impregnated epoxy material 114, commonly referred to as pre-preg.
Signal lines are primarily routed on the first and fourth layers 112 and 124, respectively. Oftentimes, signal lines must be routed from the first layer 112 to the fourth layer 124 through the reference layers 116 and 120. When this occurs, the power layer 116 and the ground layer 120 must be split, cutout, or slotted (the result is collectively termed a discontinuity) to avoid a short circuit between the signal lines being cross-routed and the reference layers. Signals routed around the cross-routed signal must necessarily be laid out across the discontinuity.
FIG. 2A is a cross sectional view of an exemplary CB 200 with a discontinuity 202. FIG. 2B is a top view of the CB 200. Referring to FIGS. 2A-B, the CB 200 includes a ground plane (or layer) 204 and voltage planes (or layers) 206 and 208. A discontinuity 202 exists between voltage planes 206 and 208. A signal 210 bridges or spans the discontinuity 202 as most clearly shown in FIG. 2B.
High-speed signals that span discontinuities in adjacent reference planes, like signal 210, generate electromagnetic (EM) radiation because of the electrical break caused by the plane discontinuity. This EM radiation adversely affects electromagnetic containment (EMC) and signal integrity (SI). For one, the discontinuity increases the electrical ground path increasing loop inductance. And the larger inductance might cause signal distortion and phase shifts.
To avoid these issues, CB designers avoid routing high-speed signals over discontinuities. But these constraints are often difficult to maintain as CB real estate shrinks or as signal density increases. Another way CB designers avoid these problems is to use stitching capacitors across discontinuities, e.g., stitching capacitor 212. The stitching capacitor 212 electrically couples the plane 208 to the plane 206 through vias 214. The stitching capacitor 212 provides alternating current (AC) coupling that reduces EM radiation at the discontinuity reducing, in turn, adverse EMC and SI effects. The addition of stitching capacitors, however, is costly. One stitching capacitor is required for each signal crossing a discontinuity. Thus, the CB component count increases, increasing cost. More components require additional CB real estate, also increasing cost.
Accordingly, a need remains for an apparatus and method of improving coupling across plane discontinuities on circuit boards.